FIG. 1(a) illustrates a generic microwave or millimeterwave integrated circuit (MMIC) package fabricated as a shielded package and containing at least two microstriplines 140 and 150. This package also includes a cover 110 and a substrate 120 with conductive sidewalls 165 which, when sealed together with a conductive seal 130, create an enclosed cavity 115 of sufficient volume to accommodate one or more MMICs. The substrate and cover are dielectric materials of relative permittivity ∈r5 and ∈r1 respectively. The cavity formed therebetween may be an air filled region where the permittivity of the air is denoted as ∈0. Package materials may include semiconductors (Si, SiGe, GaAs), ceramics (Al2O3, AlN, SiC, BeO), metals (Al, Cu, Au, W, Mo), and metal alloys (FeNiCo (Kovar), FeNiAg (SILVAR), CuW, CuMo, Al/SiC) and many others. The substrate and cover need not be made of the same materials.
The package may be shielded with conductive surfaces 160, 170 to prevent radiation from internal sources (transmitters) and to protect internal circuits from undesired coupling with fields external to the package. The conductive surfaces 160, 170 form a parallel-plate waveguide (PPW) that allows a quasi-TEM (transverse electromagnetic) mode to be supported inside the package. The TEM mode has a vertical (z-directed) electric field which propagates in any x or y direction inside the package, and has a phase velocity of (ω/c)√{square root over (∈eff)} where ω is the angular frequency, c is the speed of light in a vacuum; and, the effective dielectric constant of the PPW is given by
      ɛ    eff    =                    t        1            +              t        3            +              t        5                                      t          1                /                  ɛ                      r            ⁢                                                  ⁢            1                              +              t        3            +                        t          5                /                  ɛ          r5                    where t1, t3, and t5 are the thicknesses of the cover, air region, and substrate, respectively. A parasitic or unintentional PPW mode may be excited at discontinuities of the microstriplines such as at ends, gaps, and bends. This results in crosstalk between otherwise isolated microstriplines. The parasitic modes will also reflect at the sides of the package and result in undesired package resonances or parasitic resonances. Package resonances may exist in an empty rectangular package at frequencies near
      f    nm    =            c              2        ⁢        π        ⁢                              ɛ            eff                                ⁢                                        (                                          m                ⁢                                                                  ⁢                π                            W                        )                    2                +                              (                                          n                ⁢                                                                  ⁢                π                            L                        )                    2                    where W and L are the width and length of the rectangular package. The addition, for example, of an integrated circuit die into the package can dramatically lower the frequency of such resonances.
A conventional means of suppressing the parasitic resonances is to add lossy ferrite-loaded materials as thin layers inside the package. This is a relatively expensive method of mode suppression. Also, the ferrite layers need to be adhesively attached to a conductive surface to obtain the expected attenuation, and conducting surfaces may not be readily available inside of every package. Millimeterwave packages tend to be very small which exacerbates the assembly challenges of installing ferrite-loaded materials.